The resolution limit in traditional optical microscopy can be around 200 nm, which corresponds to the diffraction limit for visible wavelength light. To image features optically with resolution greater than 200 nm, evanescent near field interactions must be utilized. In Near Field Scanning Optical Microscopy (NSOM or SNOM), through collection of near field optical power, resolution of 50 nm can be readily achievable. In NSOM, a fiber drawn to a sharp tip with diameter less than 50 nm and coated with metal collects the near field optical power. With a more elaborate metal coated fiber, one containing a metal aperture covering a portion of the tip, lateral resolution of 20 nm is possible.
Another near-field based strategy to produce ultra-high resolution is an “apertureless near-field optical microscope”, where a metal tip replaces the metal coated glass fiber tip of ordinary NSOM. In this approach, light coming from the sample is scattered by the tip and is then collected. An apertureless NSOM can be constructed from a standard atomic force microscope (AFM) where the cantilever tip of the AFM becomes the scattering point for the light probe. Resolution of up to 3 nm can be attained, and typically can be around 50 nm
In order to improve the resolution of aperture and apertureless NSOM, researchers have investigated utilizing active tips that consist of isolated molecules or nanocrystals. These active probes can act as point dipole emitters or detectors that function with molecular resolution. However, to date the best reports show resolution limited to about 100 nm. This limit includes experiments utilizing Forster resonant energy transfer mechanisms.